The present invention relates to a plunger master cylinder for an automotive brake system, with a cylinder housing with a main bore which is open on one axial end and closed on the other axial end, with a main bore encompassing one piston entirely inside the bore and a outer piston sealing the bore from the outside at the open end. Tandem master cylinders are used as pressure generators for the actuation of the pistons in brake calipers in two brake circuits, with the pedal force exerted by the driver being boosted by a brake booster, as the case may be, and evenly distributed to the brake calipers by way of the pressure generator by axially moving the plunger pistons as to displace brake fluid from two pressure chambers in the main bore. Plunger tandem master cylinders eliminate the need for valves inside the pistons because fluid channels entering the main bore are controlled by the peripheral edges of the pistons which axially move across these channels, thereby connecting them to or disconnecting them from the pressure chambers inside the main bore.
US 20040011614 A1 shows such a plunger master cylinder. Unlike conventional plunger cylinders, the described cylinder eliminates the need for a complicated assembly. Instead of using a large-diameter end closure member and sealing devices withstanding high pressure, as they are necessary in previously known plunger master cylinders, the closed end of the main bore in the described cylinder is manufactured in one piece with the cylinder. Accordingly, the assembly is carried out from the open end of the main bore by inserting the pistons and their compression springs from this side.
Considering the high pressure generated by a master cylinder, this is a great advantage regarding cost and safety. Either just the outer piston or each piston is first connected to its respective compression spring by attaching it to a compressible spring retainer which limits the expansion of the spring. Then the spring assigned to the inner piston is inserted into the bore, followed by the inner piston, which may be connected via a retainer, and then by the preassembled arrangement of the outer spring and the outer piston held together by the retainer. Once the master cylinder is attached to the brake booster, the elements inserted into the cylinder bore are held in place. The accommodation of such a master cylinder inside a vehicle is relatively easy due to its reduced size compared to conventional plunger cylinders.
However, if the brake system needs to be filled with brake fluid prior to assembly in the vehicle, the plunger pistons can slide out of the bore due to the fact that the brake fluid is supplied under high pressure. This makes a plunger tandem master cylinder as described in US 20040011614 A1 unsuitable for such a procedure unless additional measures are taken to prevent the pistons from exiting the bore.
It is therefore an objective of the present invention to present a simple solution to make this type of plunger cylinder arrangement suitable for the so-called evacuation-and-fill process.
It is a further objective to avoid a large closing member requiring substantial efforts to properly seal its edges to withstand high pressure.